FIG. 1 is a cross-sectional view of conventional wood flooring for an under-floor heating system in which an untreated natural veneer layer is laminated on a waterproof plywood layer. As shown in FIG. 1, the conventional wood flooring has a structure wherein the natural veneer layer 24 is surface-coated with a UV coating paint to form a general UV coating layer 21′ or is resin-impregnated, and is then laminated on the waterproof plywood layer 10′ through an adhesive layer 40.
The respective layers constituting the conventional wood flooring for an under-floor heating system are briefly explained below with reference to FIG. 1.
The waterproof plywood layer 10′ is produced by layering 5 to 7 veneers together using a phenol or melamine resin adhesive in such a manner that the grain directions of the veneers are at right angles to one another, and pressing the laminate using a press.
The upper natural veneer layer 24 is produced by impregnating or unimpregnating a natural veneer with an impregnating resin, or impregnating the resin into the natural veneer under reduced or high pressure. The surface UV coating layer 21′ is produced by subjecting the upper natural veneer to top, intermediate or under coating 6˜10 times using urethane acrylate paint by common techniques.
Since the conventional flooring for an under-floor heating system comprises the waterproof plywood layer 10′ and the waterproof plywood layer 24 laminated thereon, it exhibits excellent dimensional stability against heat and moisture.
However, a conventional flooring comprising the waterproof plywood layer 10′ and the untreated upper natural veneer laminated thereon exhibits a surface scratch resistance as low as 0.5˜1N and an impact resistance as low as 10˜20 cm due to a low density of the waterproof plywood layer 10′ despite the presence of the surface UV coating layer 21′ on the untreated upper natural veneer. The scratch resistance is measured by scratching the surface UV coating layer 21′ using a diamond needle, and the impact resistance is measured by dropping a metal ball weighing 225 g onto the surface of the natural veneer. There is a large possibility that the conventional wood flooring is easily impaired when laying heavyweight household appliances on the wood flooring and using them. Accordingly, the conventional wood flooring causes consumer complaints and cannot satisfy consumer's diverse needs. There is, thus, a need for a wood flooring having improved scratch resistance and impact resistance.
Wood floorings having a surface reinforcement structure were suggested in which two melamine-impregnated overlay sheets are laminated at the upper and lower portions of a natural veneer. The prior art wood floorings have a higher surface strength than any other conventional UV-coated wood flooring, but have problems that the natural texture is poor and curls are likely to occur, which negatively affects the quality of the finished products. In addition, they show poor adhesion of the impregnated overlay layers to the veneer.
Further, a laminate wood flooring with a melamine veneer laminate wood flooring (LPM flooring) wherein a titanium dioxide (TiO2)-containing paper having a basis weight of 70˜80 g/m2 is impregnated with a melamine resin shows good surface physical properties, e.g., a scratch resistance of about 3N and an impact resistance of about 40 cm. However, the use of the titanium dioxide-containing paper for maximized resin impregnation causes poor printability despite superior impregnation performance, compared with a low-basis weight tissue paper. In addition, the application of an acrylate-type UV coating paint is impossible due to impregnation of the melamine. Furthermore, the impregnated melamine is very cold to the touch and static electricity is likely to occur due to its nature.
Volatile organic compounds (VOCs) resulting from the use of organic solvents, including formaldehyde, cause sick house syndrome, which has recently become a health problem. Under this circumstance, there is a demand for an environmentally friendly finishing material.